The present invention relates to semiconductor device manufacturing, and more particularly to a method of fabricating a metal oxide semiconductor field effect transistor (MOSFET) in which the gate and source/drain regions are independently doped in a self-aligned manner after the gate stack has been etched. The method of the present invention does not affect line width control, and no additional lithography steps are required.
In today""s most advanced semiconductor devices, the gate implant is also received by the source/drain regions. Typically, the maximum amount of dopant that the gate can receive is limited by the amount that the source/drain regions can tolerate. For example, current state-of-the-art NFETs use phosphorus for the source/drain regions. If too much phosphorus is implanted into the source/drain regions, then lateral phosphorus diffusion may be excessive causing degraded short channel effects. On the contrary, implanting high doses of phosphorus (on the order of about 5E15 cmxe2x88x922 or greater) into the gate reduces the gate depletion effect and improves the device characteristics.
In some prior art processes, wider source/drain spacers are used to accommodate a higher dose of phosphorus into the source/drain regions. However, this causes the series resistance of the transistor to significantly increase.
If arsenic is used for the source/drain doping, achieving comparable gate activation as phosphorus is difficult for the same thermal cycle. In order to achieve maximum flexibility in achieving the least poly depletion and best short channel effect control, independent doping of the source/drain regions and the gate regions is desirable.
It would thus be beneficial if a method would be developed that was capable of independent doping of the gate region and the source/drain regions. Such a method would achieve improvements in the gate region of the device without negatively impacting the source/drain regions of the device.
One possible prior art approach for independent doping of the gate and the source/drain regions includes the use of a so-called gate predoping scheme. A typical gate predoping scheme of the prior art includes the steps of:
(i) depositing polysilicon onto a surface of a gate dielectric which is formed atop a semiconductor substrate;
(ii) using a first lithographic step to block the PFET region;
(iii) implanting ions into the NFET polysilicon material;
(iv) stripping the resist employed in step (ii);
(v) using a second lithographic step to block the NFET region;
(vi) implanting ions into the PFET polysilicon material;
(vii) stripping of the resist; and
(viii) etching the gate stack region.
In this prior art process, an activation annealing step is typically performed between steps (vii) and (viii) mentioned above.
A major disadvantage of this prior art integration scheme is that the implants are performed before the gate stack has been etched. This leads to poor line width control since the P-type polysilicon will etch differently than the N-type polysilicon. Also, if the implant condition is changed, the gate etch steps needs to be re-optimized again since a different doping in the gate region will change the etch characteristics. Another major disadvantage of the aforementioned prior art gate predoping scheme is that it requires two additional lithography steps, e.g., steps (ii) and (v) mentioned-above, prior to etching of the gate region. A yet further disadvantage of this prior art process is that the different etching rates may result in recessing a portion of the substrate.
In view of the above drawbacks with prior art methods, there is a continued need for providing a method which is capable of independent doping of the gate and the source/drain regions that will allow for optimizing the doping in the gate and source/drain regions independently so that improved device characteristics can be achieved without the compromise between gate depletion and series resistance.
One object of the present invention is to provide a method of fabricating a MOSFET device which is capable of independent doping of the gate and the source/drain regions.
A further object of the present invention is to provide a method of fabricating a MOSFET device which has reduced gate depletion, improved device characteristics and limited lateral diffusion of dopant in the source/drain regions as well as the source/drain extension regions.
Another object of the present invention is to provide a method of fabricating a MOSFET device which has improved series resistance and line width control.
A yet further object of the present invention is to provide a method of fabricating a MOSFET device in which gate predoping is avoided and the number of lithographic steps is reduced.
These and other objects and advantages are achieved in the present invention by utilizing an anti-spacer structure. The anti-spacer structure of the present invention enables independent doping of the gate and source/drain regions in a self-aligned manner after the gate stack etch so that line width control is not affected, and no additional lithography steps are required. The anti-spacer structure employs a thin film (or stack of films) having poor step coverage that is deposited on the etched gate stack either before or after the source/drain regions and/or extension implants are formed.
It is noted that the lack of step coverage of the inventive anti-spacer structure enables the source/drain regions and the source/drain extensions to be blocked during gate implanting, while the sidewalls of the gate are exposed and are thus able to be implanted at an angle. The film having a lack of step coverage is referred to herein as a non-conformal film. The non-conformal film (or stack of films) may be an organic or inorganic film which may be selectively removed after the gate is implanted. The non-conformal film is thick across horizontal surfaces present in the structure, yet the non-conformal film is thin across vertical surfaces present in the structure. In some embodiments, the non-conformal film is non-existent on the vertical surfaces, i.e., vertical gate region. The variation of thickness in the non-conformal film permits the selective doping of the gate region, while blocking the source/drain regions and source/drain extensions from the gate implant.
One aspect of the present invention thus relates to a method of fabricating a MOSFET device which comprises the steps of:
(a) forming a structure having a plurality of patterned gate stacks atop a layer of gate dielectric material;
(b) forming a non-conformal film on said structure including at least said plurality of patterned gate stacks;
(c) blocking some of the plurality of patterned gate stacks with a first resist, while leaving other patterned gate stacks of said plurality unblocked;
(d) implanting first ions into said unblocked patterned gate stacks;
(e) removing said first resist and blocking said previously unblocked patterned gate stacks with a second resist;
(f) implanting second ions into said patterned gate stacks that are not blocked by said second resist; and
(g) removing said second resist and said non-conformal film.
Note that source/drain regions and source/drain extensions may be formed prior to performing step (b) above, after step (d) and step (f), or after step (g). When the source/drain regions and source/drain extensions are formed prior to performing step (b), it is necessary to form sidewall spacers on the vertical sidewalls of each patterned gate stack region. These spacers are then removed before conducting steps (b)-(g) of the present invention. In a preferred embodiment of the present invention, the source/drain extensions are formed before formation of the source/drain regions.
In the present invention, the first ions employed in step (d) may be the same or different from the second ions employed in step (f). In a preferred embodiment of the present invention, the first ions are different from the second ions. Note that in some embodiments, the ions used in steps (d) and (f) are the same, but different ion dosages are employed in each step so as to form doped gate regions having different ion concentrations.
In addition to providing a method of independent doping of the gate, source/drain regions and the source/drain extensions, the present invention also relates to a semiconductor structure, e.g., anti-spacer structure, which includes the non-conformal film thereon. Specifically, the inventive structure of the present invention, which is an intermediate structure for the final MOSFET-containing device, comprises:
a semiconductor substrate having a layer of gate dielectric material formed on a surface thereof;
a plurality of patterned gate regions formed on said layer of gate dielectric material, said plurality of patterned gate regions not being pre-doped; and
a non-conformal film formed atop exposed surfaces of said substrate or said layer of gate dielectric and said plurality of patterned gate regions, wherein said non-conformal film is thicker over horizontal surfaces, while being thinner over vertical sidewalls of each of said patterned gate regions.
Note that in some embodiments of the present invention, the patterned gate regions include a reoxidation material formed thereon.